Interesting statement:
"don't rely on UA agents for int'l COVID rules!" very TRUE
HOWEVER, you MUST rely on UA agents for UA Intl flights. The Checkin and/or Gate Agent checks your paperwork at the airport so you must rely on their interpretation (or perhaps help them a little).
But yes, at the same time, I would never trust what customer service tells you. One must research it themselves then hope that the UA agents at the airport agree with your interpretation.